As someone who worked at a startup that grew to a significant size in the "Nutraceuticals" industry (the fancy name for supplements), I can tell you that the FDA has nearly zero regulation or monitoring of supplements.
I'll omit brand names here, but I can tell you some sketchy stuff happens in supplement manufacturing all the time. During the ~6 years I worked there, only one letter came from the FDA after a whistleblower at a competitor's company came forward. The FDA sent a warning letter out to several of the large competitors in the industry to "don't do it or else" and never followed up again. The company that got in trouble got a few hundred thousand dollar fine for using mislabeled and toxic ingredients. They had one follow up inspection about 6 months after the warning and that was the end of it. For comparison that company was making ~$600M a year at the time of the fine and is now making $1B+. We carried on and never heard from the FDA again despite being equally guilty in our own company.
The guilt is what eventually led me away from the cash cow, where I went on an 18 month sabbatical to get away from any corporate greed for a little bit. I legitimately had nightmares that I would be complicity guilty of several crimes if I stayed there long enough.
I promise you, there is no oversight in supplements. There are a handful of posted guidelines. If a whistleblower comes forward the FDA might react to that single case, but they are so understaffed; the team that manages nutriceuticals is marked in the "tens" of people, not the thousands dedicated to proper medical equipment and medicines.